The present field of polymerization of monomers is varied in types of monomers, catalysts, and processes. Often, catalysts used to make polymers are expensive, may be poisoned by products of the reaction or impurities present in the monomer feed, and contain heavy metals such as chromium, mercury and cadmium that present environmental disposal problems for users. Frequently, these catalysts require the use of very high or very low temperatures and high pressures during the polymerization reaction. Separation of the catalyst from the polymer is not always possible, making the polymer less desirable for the customer.
Examples of these problems are found in the polymerization of tetrahydrofuran (THF) and dioxolane (DXL). The cationic polymerization of THF is now mostly catalyzed with a BF3/FSO3H/HClO4 mixture, or oleum. This reaction requires a promoter, typically an olefin oxide, acetyl chloride, acetic anhydride or cetene. Because of the inefficiency of the catalyst, large amounts of the catalyst and promoter are required, up to one mole of catalyst for one mole of polytetrahydrofuran (PTHF), resulting in a very expensive process. Similarly, DXL is most often polymerized using a BF3/FSO3H mixture in the presence of dichloromethane in a nitrogen atmosphere. However, the reaction is not continuous, the yield is poor, and purification of the product from the residual BF3 is difficult. As a result of these problems, polydioxolane is not manufactured on an industrial scale.
The chemical industry is always looking for new substitutes to these classical catalysts. For instance, PTHF can now be polymerized in a cost-effective and environmentally appropriate manner using antimony pentachloride as a catalyst and a mixture of carboxylic anhydrides in the presence of alcohol as a promoter. But while this new manner of catalysis has alleviated some of the processing problems, it has resulted in a polymer that, because of its black color, does not meet industry needs.
In addition, toxic catalysts often present problems in the manufacture of polymers used in medical and veterinary procedures. Those installing these polymers often desire that the polymer be metabolized by the body after the polymer has performed its function. These types of polymers are called “bioresorbable.” Many bioresorbable polymers are synthesized from lactides. These bioresorbable polymers are frequently used for suture strings, suture wire, staples, meshes, and hemostatic clamps. The polylactide are synthesized with the use of catalysts, most often a trioxide of antimony and stannous octanoate. These catalysts are toxic in even trace amounts, necessitating a careful and costly separation of the catalyst from the polymer.